Today there is a great interest in the production of anisotropic metal nanoparticles with different morphologies, the production of nanofibers being one of the most important due to their application potentials in the preparation of nanocomposites based on non-metal materials (ceramics, polymers, glasses, etc.) in order to render metal properties to these materials. Applications such as new antistatic nanocomposites, nanocomposites for shielding against electromagnetic radiation, nanocomposites and nanocomposite liquids for heat transfer, etc. make this a topic of great importance in recent technology.
The particular physiochemical properties of the AQCs, which unlike nanoparticles have electronic transition bands between different energy levels at the Fermi level (HOMO-LUMO gap or bandgap) and lack the plasmon band typical of nanoparticles, should be pointed out. These particular properties of these materials, due to the important quantum effects which characterize these AQCs, cause their behavior to be different from that of nanoparticles or massive material.
In addition, it is important to obtain the desired metal properties by introducing the lowest possible amount of metal particles in the non-metal matrices both for the cost and for not deteriorating the intrinsic properties of the matrices themselves. Due to the fact that the anisotropic geometries, such as the cylindrical-shaped fibers, allow achieving percolation with very low concentration thresholds, obtaining simple and scalable methods which allow controlling the size and shape of the nanoparticles is a current challenge of extraordinary importance.
In the last few decades, a great amount of chemical methods for the synthesis of nanoparticles of very different shapes and sizes, such as nanocylinders (Busbee, B. D.; Obare, S. O.; Murphy, C., J. Adv. Mater., 2003, 15, 414; Pérez-Juste, J.; Liz-Marzán, L. M.; Carnie, S.; Chan, D. Y. C.; Mulvaney, P., Adv. Funct. Mater. 2004, 14, 571), multishapes (Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L., J. Am. Chem. Soc., 2003, 125, 16186), nanoprisms (Pastoriza-Santos, I.; Liz-Marzán, L. M., Nano Lett., 2002, 2, 903; Millstone, J. E.; Park, S.; Shuford, K. L.; Qin, L.; Schatz, G. C.; Mirkin, C. A., J. Am. Chem. Soc., 2005, 127, 5312), nanocubes (Im, S. H.; Lee, Y. T.; Wiley, B.; Xia, Y., Ang. Chem. Int. Ed., 2005, 44, 2154), nanotetrahedrons (Wiley, B.; Herricks, T.; Sun, Y.; Xia, Y., Nano Lett., 2004, 4, 1733), or nanodiscs (Maillard, M.; Giorgio, S.; Pileni, M. P., J. Phys. Chem. B, 2003, 107, 2466) have been developed.
Even though this has been one of the most spectacular advances of colloidal chemistry, the methods developed up until now however suffer from various problems and complexities (Pérez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzán, L. M.; Mulvaney, P., Coordination Chemistry Reviews, 249, 2005, 1870-1901) which make them practically unviable for scaling and producing anisotropic nanoparticles in a simple and controlled manner (Jin, R.; Cao, Y. C.; Hao, E.; Metraux, G. S.; Schatz, G. C.; Mirkin, C. A., Nature, 2003, 425, 487).
Apart from other methods that are not relevant to the objectives of the present invention (such as electrochemical deposition methods using solid patterns, etc.) the chemical methods developed up until now to achieve anisotropic growth of the nanoparticles are based on:
The use of agents such as polymers, surfactants, etc. which, by preferential absorption on any of the crystallographic faces of the metal, inhibit the growth of this/these face/faces thus achieving an anisotropic growth of the nanoparticles. Although the control may be suitable in some particular cases, amounts of nanoparticles at very low concentrations are always obtained and in any event in order to achieve the control, complicated reaction conditions such as high temperatures or organic solvents like those used in U.S. Pat. No. 7,585,349, must be used, or the use of very poorly defined and therefore very poorly scalable processes consisting of the complex combination of seeds (crystallization cores), surfactants, addition of heavy metal salts, processes in multiple steps, etc. (Jana, N. R.; Gearheart, L.; Murphy, C. J., Adv. Mater. 13, 1389, 2001; Christopher J. Johnson, Erik Dujardin, Sean A. Davis, Catherine J. Murphy, Stephen Mann, J. Mater. Chem. 12, 1765, 2002; Zhi-Chuan Xu, Cheng-Min Shen, Tian-Zhon Yang, Huai-Ruo Zang, Hong-Jun Gao, Nanotechnology, 18, 115608, 2007) as well as the use of processes in hydrothermal conditions, at pressures and temperatures above that of the environment also in the presence of different inhibitory substances of the specific growth of crystallographic faces (polymers, surfactants, etc) (Perez-Juste, J. et al.). In this last case, the shape and sizes can be very precisely controlled, but the method suffers from the drawbacks of using pressurized reactors. The mentioned methods are extremely sensitive to the experimental conditions, mentioning by way of example that simply changing the container that supplies the surfactants decisively affects the final shape being able to be truly anisotropic (Smith, D. K.; Korgel, B. A., Langmuir, 2008, 24, 644-649). Furthermore, Barnard et al. (Alireza Seyed-Razavi, Ian K. Snook, Amanda S. Barnard, J. Mater. Chem, 2010, 20, 416-421) recently conclude that a complete theory of nanoparticle evolution does not yet exist.
For all these reasons, there is a current need to provide chemical methods which allow control in the formation of anisotropic particles in a simple manner.